Conventional acoustic resonator filters, such as ladder filters, are formed of series resonators connected in series between an input terminal and an output terminal, and shunt resonators respectively connected between at least one of the series resistors and a ground voltage. Each of the series resonators and the shunt resonators may be a bulk acoustic wave (BAW) resonator, such as a thin film bulk acoustic resonator (FBAR) or a solidly mounted resonator (SMR), or a surface acoustic wave (SAW) resonator, having a corresponding parallel resonance frequency Fp and series resonance frequency Fs.
Acoustic resonator filters may be used for various applications, particularly for portable communication devices, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers and the like, configured to communicate over wireless networks. When a portable communication device includes a transmitter and receiver (or transceiver), typically connected to a common antenna, for sending and receiving data and control signals over the wireless network, acoustic resonator filters may be incorporated in a duplexer for filtering both transmit and receive signals, respectively, at different frequencies and filter bandwidths.
However, filter bandwidth ranges of acoustic resonator filters are generally restricted by various factors, such as limited ranges of electromechanical coupling coefficients Kt2, limited material quality factor (Q-factor), and fabrication process limitations (e.g., electrostatic discharge (ESD) issues). Accordingly, there is a need for acoustic filter bandwidths with improved filter characteristics, such as higher coupling coefficients Kt2, and/or narrower acoustic filter bandwidths while maintaining current filter characteristics.